--- Original report ---
Some authoritative DNS servers and/or mitigation devices/software silently drop queries that have uppercase letters in them. Furthermore, the clarification of the case insensitive comparison in the following two sentences after that particular sentence use the term MUST. I suspect some readers of the RFC are reading the word "should" and aren't reading the rest of the paragraph.

---- WK Update ----
The full quote is: "According to the original DNS design decision, comparisons on name
lookup for DNS queries should be case insensitive [STD13]. ", and the title of this (RFC4343) is "Domain Name System (DNS) Case Insensitivity Clarification" -- seeing as the whole point of this document is to clarify the original spec, I think that readers will read the RFC2119 bits.

However, I do agree that this could be better worded, and future updates of this document should probably reword this to make it clearer.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

It should say:

The key words "MUST" and "MAY" in this document are to be
interpreted as described in [RFC2119].

Notes:

Other than in the above-quoted sentence, there are no
instances of "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", SHOULD",
"SHOULD NOT", "RECOMMENDED", or "OPTIONAL" in the RFC (and the
instances above surely cannot be interpreted as described in RFC
2119; they are mere labels in the context of that sentence).
--VERIFIER NOTES--
The keyword paragraph is standard, and although words are mentioned that are later not used, this is not an error.